On 9 April 2015 at 16:11, Jussi Kalliokoski <[email protected]> wrote: > On Thu, Apr 9, 2015 at 4:04 PM, liorean <[email protected]> wrote: >> >> Do we really need it? >> Your «foo(1, ?, 2);» is equivalent to «a=>foo(1,a,2)». >> Your «foo(?, 1, ???);» is equivalent to «(a,...b)=>foo(a,1,...b)». >> Your «foo(1, ???, 2);» is equivalent to «(...a)=>foo(...[1,...a,2])». > > > Not exactly. Using the placeholder syntax, `this` remains context dependent, > whereas with your examples you get `null` as `this`.
No, «this» is lexically bound to be that of the enclosing lexical scope in arrow functions, so it would be whatever that is. But that doesn't really matter as the function call to «foo» doesn't use the «this» of the arrow function. Now, if we were to say your «foo» were actually «foo.bar», and you did the same replacement in the arrow function, the «this» value of the «bar» call would be «foo», so that's pretty much what is wanted as well. The case where this breaks is if you were to replace only the «bar» method with the arrow function, in which case it would use the lexical «this» instead of «foo», but that's obviously not the right transformation to use. > This might not seem like such a big deal until you consider it in > combination with the proposed bind syntax [1]. > > Also in your examples, redefining `foo` will lead to different results. The > placeholder syntax has a lot more room for optimization in the JIT compiler > (the partially applied result is guaranteed to have no side effects for > example, so the compiler can create a version of the original function where > it can inline the specified arguments; less moving parts, easier to > optimize). Yeah, it's susceptible to that problem, yes. Do you want me to fix that for you if you really want it? Your «foo(1, ?, 2);» is equivalent to «((f,a)=>f(1,a,2))(foo)». Your «foo(?, 1, ???);» is equivalent to «((f,a,...b)=>f(a,1,...b))(foo)». Your «foo(1, ???, 2);» is equivalent to «((f,...a)=>f(...[1,...a,2]))(foo)». I guess I didn't think of these cases though, because I only use explicit arguments to my functions these days, I never use the «this» keyword. If I want a function to operate on an object, I pass that object into the function. I also try to not reuse my variables unless they are part of an iteration, in which case they are always local variables that are only handled in the iteration process itself. But that's a side issue, as it's about my code rather than precepts of the language. -- David "liorean" Andersson _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
